1. Field of the Invention
The invention relates to steel making and more particularly relates to improving stability of the slag by-product of steel making.
2. Brief Description of the Prior Art
As is well known, converter slag is a highly basic slag by-product of a converter steel making plant. Table 1 below shows the chemical compositions of converter slag and blast furnace slag, their basicity and melting points. As is seen in the Table 1 basicity of converter slag (CaO/SiO.sub.2) is as high as 2.5 - 4.7. This is a higher range than that of blast furnace slag. Also as shown the chemical composition of converter slag, particularly calcium dioxide, silicon dioxide and iron contents may vary largely depending on operating conditions under which it is formed.
Table 1 ______________________________________ Chemical composition and melting points of blast furnace slag and converter slag. Chem. Comp. Blast Furnace Slag Converter Slag ______________________________________ CaO 40 - 43% 35 - 59% SiO.sub.2 32 - 36% 10 - 18% Al.sub.2 O.sub.3 12 - 18% 0.5 - 1.5% Fe* (Total) 0.2 - 1.2% 8.0 - 25.0% MgO 2.0 - 7.0% 1.2 - 4.0% MnO 0.5 - 2.0% 1.0 - 8.0% TiO.sub.2 0.5 - 2.2% 0.5- 1.5% P.sub.2 O.sub.5 0.02 - 0.10% 1.5 - 3.0% S 0.70 - 1.50% 0.06 - 0.20% F tr 0.3 - 0.9% CaO/SiO.sub.2 Basicity 1.1 - 1.3 2.5 - 4.7 Melting Point .degree. C 1,360.degree. - 1,430.degree. 1,450.degree. - 1,630.degree. ______________________________________ *The percentage of Fe, sometimes referred to hereinafter as "Total Fe" or "Total iron" and as found throughout the specification and/or used in the claims, means the iron content contributed by ferrous oxide and ferric oxide. It does not include small iron particles physically mixed in the slag.
As shown in Table 1, the melting point of converter slag is 1,450.degree. C. -1,630.degree. C. or 80.degree. C. - 130.degree. C. higher than the melting point of blast furnace slag (1,360.degree. C. - 1,430.degree. C.). Therefore, fluidity of the converter slag is inferior to blast furnace slag fluidity at the same temperature. In fact, converter slag is considered a very viscous slag. Because of its relatively high melting point, converter slag held in a slag ladle easily forms a solidification layer at its surface. Solidification at the contact surface between the ladle inside wall and the molten slag also usually proceeds speedily. Further, even when thin multiple castings of slag is carried out in a dry pit, thin slag flow is difficult due to poor fluidity of the converter slag and results in formation of a partially lump type slag.
Prior hereto, only converter slag of good fluidity was granulated in conventional granulation equipment. The amount of converter slag which could be safely granulated is only 20 to 40% of the total slag. The operation is very dangerous, often resulting in an explosion. The cause of the explosion phenomenon is not clearly understood. Theories of the cause include the following. First the temperature of converter slag at the time of granulation is 1,500.degree. C. - 1,650.degree. C. This temperature is very near to the melting point of fine metallic iron particles usually found dispersed in molten slag as well as very near to the melting point of comparatively large metallic iron grains poured into the ladle and inter mixed with the slag at the time of slag tapping. When this metallic iron comes into contact with cooling water the following reaction proceeds very rapidly: EQU Fe + H.sub.2 O .fwdarw. FeO + H.sub.2
the hydrogen gas formed reacts explosively in air. Secondly, since the fluidity of converter slag is very poor compared to blast furnace slag, the flow rate of the converter slag from the ladle is not uniform and part of the slag becomes the semi-molten lump type in water. Water surrounding the lump and water drawn into the lump rapidly vaporizes from the latent heat of the lump; rapid expansion of the vapor leads to decomposition. At the same time due to the presence of metallic iron particles contained in the lump, decomposition will proceed readily. Large explosions may be caused by a combination of the effects of these causes. At any rate, explosions during granulation of the converter slag indicates clearly that the explosion has a close relationship with the presence of metallic iron particles in the slag and with slag fluidity.
As mentioned above, fine metallic iron particles are usually found solidified in converter slag. These iron particles are not included in the Fe percent shown in Table 1, supra. During oxygen blowing in the converter, fine metallic iron particles are sprayed with the oxygen jet into the molten slag, and are physically dispersed, then solidified in the slag. Therefore these fine metallic particles are always observed in the converter slag tapped. Usually the proportion of these metallic iron particles is within the range of from 2 to 10% by weight of the converter slag.
The fine metallic iron particles observed in solidified converter slag are oxidized with time to show the characteristic ferruginous red-brown color. These iron particles eventually fall off the slag surface, giving an undesired effect to the converter slag, limiting its utilization. Although some differences are observed depending on cooling speed and treating method, slaking phenomenon caused by chemical change of free lime in the converter slag is observed with time, both in the case of lump type slag and in particle type slag. Dusting phenomenon is also observed, caused by expansion of slag from the inside. This occurs with aging. Also when the converter slag contacts rain water, large amounts of free lime are leached away, dissolved in the water. This is not always desirable for the environment. Apparently quick lime charged into the converter during steel making is not necessarily completely slagged, but quick lime is known to be uniformly dispersed in the slag microscopically.
After surveying actual converter operation, we confirmed that there was a close relationship between slag basicity and unslagged lime content. With the increase of slag basicity CaO/SiO.sub.2, unslagged quick lime content is increased and if the basicity falls below 2.0, unslagged quick lime is not readily observed. Since the basicity of converter slag is as high as 2.5 - 4.7, formation of complex compounds consisting mainly of 3CaO.SiO.sub.2 (basicity 2.8) or calcium ferrites (CaO.Fe.sub.2 O.sub.3 or 2CaO.Fe.sub.2 O.sub.3) among various oxides such as CaO, SiO.sub.2, Al.sub.2 O.sub.3, FeO, Fe.sub.2 O.sub.3, MgO, P.sub.2 O.sub.5 occurs. These compounds are said to be decomposed into complex compounds mainly consisting of 2CaO.SiO.sub.2 (basicity 1.86) and free lime during the process of cooling the slag. In the second process, 2CaO.SiO.sub.2, which is the main composition of the complex compounds, transform .alpha..fwdarw..alpha.'.fwdarw..beta..fwdarw..gamma. in the cooling process and is said to become stable .gamma. 2 CaO.SiO.sub.2 at room temperature. In the transformation from .beta. type to .gamma. type, a volume increase is accompanied therefore by the expansion of the slag causing the dusting phenomenon.
Compositions like MnO and P.sub.2 O.sub.5 have the effect of preventing transformation of .beta. type to .gamma. type, but FeO is said to have an effect of helping the above transformation. Therefore the fact that total Fe is high in the converter slag means that the slag is in the condition of encouraging the dusting phenomenon.
It is very difficult to differentiate between free lime and unslagged lime content in the practical converter slag. The lime content may be causing the undesired slaking phenomenon by forming stable calcium hydroxide [Ca(OH).sub.2 ] in contact with water.
Although the above are theories not yet proved, converter slag has basic properties which result in the undesired dusting phenomenon by expansion and the undesired slaking phenomenon by contact with water. These are detrimental to effective utilization of the converter slag.
Our invention provides a method which solves the problems described above as associated with converter slags. That is, by this invention the basicity of the slag is lowered, composition fluctuation is reduced, the melting point of the slag is lowered as much as possible and its fluctuation is decreased. At the same time, iron particle content in the slag is lowered. The result is a reduction in the defects associated with basicity of the slag and iron particle content.
In the past in order to improve the chemical composition of converter slag a method of making synthetic slag of desired chemical composition by adding powder materials was considered. However, the above method is disadvantageous in that large amounts of energy are required to melt the solid addition materials.
Our invention offers a means of using converter slag by removing its basic defects. The invention can also be applied widely to ferruginous metallurgical slags such as the slag by-product of electric steel making and open-hearth furnace processes.